I've actually seen longer waits for Soarin' than Test Track - so yes, flip a coin UNLESS you can use single rider. TT has single rider. In that case, get FP for Soarin.
Really, if you do rope drop for one and FP+ for the other, you should be fine. Back before the days of FP+ that's what happened- half the rope droppers went to one and half to the other. So nothing has probably changed in that respect!